Characterizing Photosymbiosis Between Fraginae Bivalves and Symbiodinium Using Phylogenetics and Stable Isotopes

Abstract

Photosymbiotic associations between heterotrophic hosts and photosynthetic algae play crucial roles in maintaining the trophic and structural integrity of coral reef ecosystems. The marine bivalve subfamily Fraginae contains both non-symbiotic and photosymbiotic lineages, making it an ideal comparative system to study the origin and evolutionary adaptations of photosymbiosis. The symbiotic species exhibit unique morphological adaptations to photosymbiosis. However, the basic biology of these photosymbiotic relationships, such as symbiont diversity and nutritional benefits, has not been thoroughly characterized. In this study, we examined the general morphology of four Fraginae species occupying different depths (0-10 m): Corculum cardissa, Fragum fragum, Fragum scruposum, and Fragum sueziense. Abundant symbionts were found in the mantle, gill, and part of the foot, contained in tubular networks within host tissues. We used molecular phylogenetics to investigate the algal symbiont community of these Fraginae species. Results showed that symbionts from all four species are dinoflagellates belonging to the Symbiodinium clade C and we did not detect any host-specific or geographic-specific genetic structures within the symbionts. We also used stable carbon isotope analyses to examine whether the cockles are directly utilizing photosynthetically derived carbon sources. All species show less depleted 13 C compared to filter-feeding bivalves, suggesting at least part of their organic carbon is derived directly from the symbionts. However, 13 C depletion of Fragum sueziense collected from deeper habitats are less distinguishable from filter-feeding bivalves. This indicates that species in deeper habitats may rely less on photosymbiosis due to the reduced light availability. Given that the symbiotic fragines exhibit varying morphologies, habitats, and utilization of symbiont photosynthesis, the subfamily represents an ideal model system to study differential adaptations to photosymbiosis.

Shedding light: A phylotranscriptomic perspective illuminates the origin of photosymbiosis in marine bivalves

“…In contrast with the relatively labile shell and mantle adaptations, it is possible that the evolution of host metabolomic and developmental Therefore, it is highly likely that host-symbiont interactions and reliance vary greatly among hostsymbiont pairs, depending on symbiont types, as well as degrees of host adaptations. Li et al 2018 [8] summarized known symbiont diversity in giant clams and showed that several fragine species harbor symbionts from the genus Cladocopium, which also occupies cnidarians, giant clams, and foraminifera. However, this by no means captured the full spectrum of potential symbiont diversity in photosymbiotic bivalves, especially when new host species are discovered regularly (e.g., [34]).…”

Shedding light: A phylotranscriptomic perspective illuminates the origin of photosymbiosis in marine bivalves

“…better isotopic metrics, Therefore, it is highly likely that host-symbiont interactions and reliance vary greatly among hostsymbiont pairs, depending on symbiont types, as well as degrees of host adaptations. Li et al 2018 [8] summarized known symbiont diversity in giant clams and showed that several fragine species harbor symbionts from the genus Cladocopium, which also occupies cnidarians, giant clams, and foraminifera. However, this by no means captured the full spectrum of potential symbiont diversity in photosymbiotic bivalves, especially when new host species are discovered regularly (e.g., [63]).…”

“…Giant clams acquire a considerable amount of organic carbon through photosymbiosis [10]. All species in the subfamily possess symbiotic algae from the family Symbiodiniaceae (also found in symbiosis with corals), mostly placed in the genera Symbiodinium, Cladocopium, and Durusdinium [8,11]. The giant clam harbors extracellular symbionts within a tubular network derived from the digestive system that extends into the mantle tissue [12][13].…”

“…The photosymbiotic species are exclusive to three genera: Fragum, Corculum, and Lunulicardia, comprising at least 23 species [7]. Part or most of their energy source is derived from algal photosynthesis [8]. Symbiont diversity within Fraginae is less explored, but species examined so far harbor algae belonging to Symbiodiniaceaegenera Symbiodinium and Cladocopium [8].…”

“…Part or most of their energy source is derived from algal photosynthesis [8]. Symbiont diversity within Fraginae is less explored, but species examined so far harbor algae belonging to Symbiodiniaceaegenera Symbiodinium and Cladocopium [8]. Photosymbiotic fragines host algae within not only the mantle, but also the gill and part of the foot.…”